1. Field of the Invention
The present invention relates to an electrically rewritable nonvolatile semiconductor device (EEPROM) and particularly to an EEPROM in which a multi-value memory larger than one bit is stored in one memory cell.
2. Description of the Related Art
As one of electrically rewritable nonvolatile semiconductor memory devices (EEPROM), an NAND type EEPROM in which a high integration can be made is known. In this type of EEPROM, each of a plurality of memory cells has an n-channel FETMOS structure having a floating gate, as a charge storage layer, and a control gate. Then, adjacent memory cells have a source and a drain in which are shared, and is connected in series. The memory cells connected in this manner are to a bitline as one unit. A memory cell array is integrated on a p-type substrate or a p-type well formed on an n-type substrate. The drain side of an NAND cell is connected to the bitline through a select gate, and the source side is connected to a common source line through the select gate. The control gates of the memory cells are sequentially formed in a row direction, and used as a word line.
FIG. 1A is a plan view showing one NAND cell portion of the memory cell array, and FIG. 1B is an equivalent circuit diagram. FIG. 2A is a cross-sectional view taken along line of 2A--2A of FIG. 1A, and FIG. 2B is a cross-sectional view taken along line of 2B--2B of FIG. 1A.
A memory cell array having a plurality of NAND cells is formed on a p-type silicon substrate 11 (or p-type well), which is surrounded with a device isolation oxide film 12. In this example, one NAND cell comprises eight memory cells M1 to M8, which are connected in series. Each of the memory cells comprises a floating gate (14.sub.1 to 14.sub.8) and a control gate 16 (16.sub.1 to 16.sub.8). Each floating gate 14 is formed on the substrate 11 through a tunnel insulating film 13, and each control gate 16 is formed thereon through a gate insulating film 15. An n-type diffusion layer 19 is used as both source and drain, and the adjacent memory cells use the n-type diffusion in common so as to be connected in series.
First select gates 14.sub.9 and 16.sub.9 are provided at the drain side of the NAND cell, and second select gates 14.sub.10 and 16.sub.10 are provided at the source side of the NAND cell. The first select gates 14.sub.9 and 16.sub.9 are formed at the same time with the floating gates of the memory cells and the control gates. The above-formed substrate is covered with a CVD oxide film 17, and a bitline 18 is formed thereon. The control gates 16 of the NAND cell are provided as control gates CG1 to CG8 in common. These control gates are used as word lines.
The select gates 14.sub.9, 16.sub.9 and 14.sub.10, 16.sub.10 are sequentially formed in a row direction, and used as select gate SG1 and SG2, respectively.
FIG. 3 shows an equivalent circuit of the memory cell array having such NAND cells arranged in a matrix form. One source line per 64 bitlines is connected a substrate voltage line of Al, polysilicon, etc., through a contact. The substrate voltage line is connected to a peripheral circuit. The control gates of the memory cell and the first and second select gates are sequentially formed in a row direction. Normally, a set of memory cells, in which a plurality of the memory cells are connected to the control gates, is called as one page. Then, a set of pages, which are sandwiched between a pair of select gates (first selection gate on the drain side and second select gate on the source side) is called one NAND block or simply one block.
An operation of an NAND cell type EEPROM will be explained as follows.
The data write operation is performed from the memory cell, which is far from the bitline, in order. A boosted voltage Vpp (=about 20V) is applied to the control gate of the selected memory cell. An intermediate voltage Vm (=about 10V) is applied to the control gates of the other non-select memory cells and the first select gate. 0V ("0" state) or an intermediate potential ("1" state) is applied to the bitline in accordance with data. At this time, the potential of the bitline is transmitted to the select cell. At the time of "0" state, a high voltage is applied to the floating gate of the select memory cell and the substrate. As a result, an electron is tunnel-injected to the floating gate from the substrate, and a threshold voltage is moved to a positive direction. At the time of "1" state, the threshold voltage is unchanged.
Data erasing is performed by the block unit at substantially the same time. Specifically, all control gates of the blocks to be erased and the select gates are set to "0" state, and a boosted voltage VppE (about 20V) is applied to a p-type well and an n-type substrate. VppE is also applied to the control gates of the blocks and the select gates, which no erasing is performed. As a result, in the memory cells of the erasing blocks, the electron of the floating gate is discharged to the well and the threshold voltage is moved to the a negative direction.
A data read operation is performed as follows.
Specifically, the control gate of the selected memory cell is set to "0" state, and the control gates of the other memory cells are set to a power-supply voltage Vcc (e.g., 3V). Then, the data read operation is performed by detecting whether a current flows into the select memory cell or not. In a NAND cell type EEPROM, since the plurality of memory cells are connected in a cascade form, an amount of cell currents is small at the reading time. Moreover, since the control gate of the memory cell and the first and second select gates are continuously arranged in a row direction, data for one page is simultaneously read to the bitline.
Because of the limitation of the read operation, the threshold voltage after write "1" state must be controlled to be between 0V to Vcc. Due to this, a verify write is performed, only a memory cell in which "1" is insufficiently written is detected. Then, rewrite data is provided to execute a rewrite operation of only the memory cell in which "1" is insufficiently written (verify write of each bit). The selected control gate is set to, for example, 0.5V (verify voltage) to execute the verify read, so as to detect the memory cell in which "1" insufficiently written.
In other words, if the threshold voltage of the memory cell is not 0.5V or more, the current flows into the select memory cell, and the memory cell is detected as the memory cell in which "1" is insufficiently written. The current naturally flows into the memory cell of "0" state. Due to this, a verify circuit for compensating for the current flowing into the memory cell is provided so that the memory cell is not erroneously recognized as the memory cell in which "1" insufficiently is written. A write verify is performed at a high speed by the verify circuit.
The write and the write verify operations are repeated to execute the data write operation, so that write time of each memory cell is optimized, and the threshold voltage after write "1" must be controlled to be between 0V to Vcc.
In the above-explained NAND cell type EEPROM, there is proposed a multi-value memory cell storing three data "0", "1", "2" or more in a state after write operation (FIG. 4A). In this case, for example, in a "0" state, the threshold voltage is set to be negative. In a "1" state, the threshold voltage is set to from 0V to Vcc/2. In a "2" state, the threshold voltage is set from Vcc2 to Vcc.
FIG. 4B shows a conventional verify read operation to check whether or not write operation is sufficiently is performed in this type of ternary memory cell.
In the write operation, after a write voltage (Vpp) is applied to the control gate of the memory cell (steps S1, S2), a first verify read cycle (step S4) and a second verify read cycle (step S5) are sequentially performed. The first verify read cycle checks whether or not "2" state is sufficiently written, and the second verify read cycle checks whether or not "1" state is sufficiently written. A write pulse is applied to the memory cell in which data is insufficiently written (step S7). Thus, the verify first cycle, the verify second cycle, and the rewrite are repeated until all memory cells are sufficiently written (steps S4 to S7).
The steps of the above-mentioned write operation are the same as the case of a four-value memory cell shown in FIGS. 5A and 5B. Specifically, at the time of a verify read operation second cycle (step S4), and third verify read cycle (step S5) are sequentially performed. The first verify read cycle checks whether or not "3" is sufficiently written (step 3), the second verify read cycle checks whether or not "2" is sufficiently written, and the third verify read cycle checks whether or not "1" is sufficiently written.
However, in this type of EEPROM, the following problems exist in write operation.
Specifically, for example, in the ternary memory cell, data "1" whose write threshold value is small is sufficiently written. Thereafter, "2" is sufficiently written. According to the conventional write method, in the memory cell in which "1" is written, after the memory cell in which "1" is written is sufficiently written, the unnecessary second verify read cycle for checking whether or not "1" is sufficiently written is performed until the writing of "2" is completed. As a result, verify read time is increased, and the entire write time is increased.
In the case of the four-value memory cell, data "1" whose write threshold value is small is first written. Thereafter, "2" write operation is sufficiently written, and "3" is sufficiently written. According to the conventional write method, in the memory cell in which "1" is written, after "1" is sufficiently written, the unnecessary second verify read cycle for checking whether or not "1" is sufficiently written is performed until the writing of "2" and "3" are completed. Then, in the memory cell in which "2" is written, after "2" is sufficiently written, the unnecessary second verify read cycle for checking whether or not "3" is sufficiently written is performed until the writing of "3" is completed. As a result, verify read time is increased, and the entire write time is increased.
In the conventional nonvolatile semiconductor memory device (EEPROM), the following problems other than the above-mentioned problems peculiar to the multi-value storable EEPROM exist.
Specifically, in the NAND cell type EEPROM, the control gate of the memory cell selected at the time of data reading is set to 0V, and the control gates of the other memory cells is set to Vcc (e.g., 3V) to detect whether or not a cell current Icell flows. In this case, the amount of the cell current is influenced by not only the threshold voltage to be read but also the threshold voltages of all residual cells connected in series. In the case of one NAND cell comprising eight memory cells connected in series, all threshold voltages of eight cells connected in series are in a negative state ("1" state) if the amount of the cell current Icell (best) is the largest (the amount of resistance is the smallest). If the amount of the cell current Icell (worst) is the smallest (the amount of resistance is the largest), the memory cell (e.g., MC1 of FIG. 3) of the first bitline contact side is read as "1" when the threshold voltages of the other cells connected to the read cells in series are in a positive state ("0" state).
The cell current flows to the source line from the bitline through the memory cell. In the conventional memory cell array, the source is shared in the NAND cell for one page to be read at the same time (FIG. 3). For reading the memory cell, which is the farthest from the contact between the source and the substrate voltage line (memory cell MC1 of FIG. 3), the following case is assumed.
Specifically, the threshold voltages of the other seven cells connected to the memory cell MC in series, and the resistance of the other NAND columns having the source in column is minimum (the amount of the cell current (best) is the largest). In this case, at an initial read time, the cell current flows from the small NAND column, and the resistance of the source line is large. As a result, the potential of the source line of the NAND cell to which the memory cell MC1 belongs is set to I (I: cell current flowing at the initial read time, R: resistance of the source line).
In other words, the source of the memory cell of the NAND column including the memory cell MC floats from a ground potential Vss. As a result, the voltage between the source and the drain of the memory cell and the voltage between the source and gate. Moreover, since a substrate bias effect occurs due to the float of the source from Vss, conductance of the memory cell of the NAND cell column including MC1 is reduced. Thus, since the source line floats from the ground potential when the resistance of the source line is large, the cell current does not easily flow in the NAND column in which the amount of the cell current is small.
To read a negative threshold voltage of the memory cell, "1" state, it is needed that the bitline potential be reduced from a precharge potential by .DELTA.VB. The maximum value of bitline discharge time TRWL is determined when the amount of the cell current is the smallest. In a case where the source line does not float, TRWL=CB (bitline capacitance)/Icell (worst). In the above-mentioned memory cell, since the source line floats, TRWL is further increased, and random access time is increased.
In the conventional NAND cell type EEPROM, the bitlines having the same number as the number of the memory cell columns are provided in the column direction as shown in FIG. 3. There is possibility that the size of the memory cell will be reduced in the column direction in the future by a trench device separation technique (Aritome et. al., IEDM Tech. Dig. pp. 61 (1944)) and the like. Due to this, it will be difficult to process the bitline by the same pitch as the memory cell column.
For storing multi-value data in the conventional NAND cell type EEPROM and executing the verify write of each bit by the verify circuit, the following verify read cycles are performed until the writing of all data is completed.
For example, at the time of the verify read, in the ternary memory cell, two verify read cycles are performed, and in the four-value memory cell, three verify read cycles are performed. As a result, verify read time is increased, and the entire write time is increased.
In the conventional EEPROM, since the source line floats from the ground potential, the bitline discharge time is increased, and the random access time is also increased. Moreover, in the conventional EEPROM, the bitlines having the same number as the number of the memory cell columns are provided in the column direction. However, if the size of the memory cell is reduced in the column direction by the trench device separation technique, it will be difficult to process the bitline by the same pitch as the memory cell column.